The Chromecast app category in the Play Store is unsurprisingly being renamed to the "Google Cast" section, getting ready for new devices that support the protocol. If you'll recall back to the announcement of Android TV, more and more devices are going to be supporting the Google Cast protocol going forward to let you sling content from your mobile devices to other entertainment boxes.
The branding helps clear up confusion, and starts drawing a line between Chromecast, the streaming TV-connected device, and Google Cast, which is the way different devices talk to each other and hand off content.
Nothing monumental changing here other than the name of a button, and it's a change we expected to happen as the commercial release of Android TV nears. If anything, this indicates that we could be seeing more devices that support the Google Cast system released sooner rather than later.